Can Collada export vertex colors?
As far as I know, blender does not support exporting vertex colors to COLLADA (yet).

The only other formats that blender can export to (with the official export scripts, it would be perfectly possible to write your own exporter in python)
which support vertex colors are ascii fbx and ply.

If fbx doesn't work for you, you could try joining all your objects into one object and then exporting to ply:

Select the objects which you want to join/export a press CtrlJ, then export as ply in Info panel > File > Export > Stanford (.ply).